In optical head devices for optical disks, a single lens having an aspherical surface commonly is used as an objective lens for recording information or reproducing recorded information by focusing a light beam onto a diffraction-limited spot on an information recording surface of the optical disk.
In the following, a conventional optical head device will be described, with reference to an accompanying drawing.
FIG. 7 schematically shows a configuration of the conventional optical head device. As shown in FIG. 7, a light beam emitted from a semiconductor laser 151 is subjected to a change in direction of its optical path by a beam splitter 152, and turned into substantially parallel light by a collimator lens 153. The direction of the optical path of this light beam further is changed by a mirror 154 for bending the optical path, then this light beam is focused on an information recording surface 157 of an optical disk 156 by an objective lens 155. This objective lens 155 is driven by an actuator 160. The light beam that has been focused on the information recording surface 157 of the optical disk 156 is diffracted by roughness formed on the information recording surface 157. The light beam that has been reflected and diffracted by the information recording surface 157 of the optical disk 156 passes through the objective lens 155, is subjected to the change in the direction of the optical path by the mirror 154 for bending the optical path, passes through the collimator lens 153, the beam splitter 152 and a cylindrical lens 158, and is focused on a photo detector 159. Based on an electrical signal of the photo detector 159, the change in light quantity that has been modulated by the information recording surface 157 of the optical disk 156 is detected, thereby reading data on the information recording surface 157.
In the objective lens 155, a wavefront aberration may be generated because of a manufacturing error during manufacturing. This wavefront aberration theoretically can be divided into aberration components of a third-order spherical aberration, a third-order coma aberration, a third-order astigmatism and a high-order aberration.
Among these aberration components, the third-order coma aberration can be avoided by designed by forming a lens surface of the objective lens 155 to have a rotationally symmetric aspherical surface. However, in reality, decentration (displacement amount in a direction perpendicular to an optical axis) between a first surface 161 of the objective lens 155 on a parallel beam side and a second surface 162 thereof on a focusing side and tilt (inclination with respect to a surface perpendicular to the optical axis) of the first surface 161 of the objective lens 155, the second surface 162 thereof, or both surfaces are generated during manufacture and serve as predominant factors in generating the third-order coma aberration. Out of these two factors, the third-order coma aberration that is caused by the tilt is proportional to substantially the third power of NA (numerical aperture) of the objective lens 155.
In general, when a lens is inclined with respect to an optical axis, the third-order coma aberration is generated. Thus, by adjusting the angle of inclination, the generated third-order coma aberration can cancel out the third-order coma aberration due to the manufacturing error. For this purpose, the objective lens 155 of the optical head device is inclined during an assembly process of the optical head device, so as to reduce the third-order coma aberration. Such an operation is called “a tilt adjustment,” and this angle of inclination is called “a tilt adjustment angle.”
In recent years, an increasingly higher recording density has been achieved as seen in DVDs (digital video disks) or DVD-RAMs, for example. The key to achieving the densification is how small a spot can be formed on an optical disk, and it is known that the spot diameter can be reduced by increasing the numerical aperture of the lens. Accordingly, in order to achieve higher density, the numerical aperture of the lens gradually has become larger and larger, and now is 0.6 in DVDs. For a further densification, the numerical aperture has to be 0.6 or larger. Considering a manufacturing error during the manufacture of the lens or an assembly error during assembling on the optical head, lenses are designed conventionally so that the generation of decentration of the first surface and the second surface of the lens does not increase aberration and that the incidence of abaxial light does not increase aberration. In spite of a great demand, lenses with a larger numerical aperture have not been commercialized yet partly because, though it is easy to reduce only axial aberration in a lens design, it is very difficult to design a single lens having a sufficient tolerance while taking a manufacturing tolerance and an assembly tolerance into account.
Also, when the numerical aperture is raised, the third-order coma aberration that is caused by the inclination of the disk becomes larger. At present, there is a possibility that warping of the disk causes the inclination of about 0.5°, which generates the third-order coma aberration of about 70 mλ in an optical system with a numerical aperture of 0.6 and a focal length of 3.0 mm. In order that a spot is sufficiently small and reproducible in a DVD system, the third-order coma aberration is required to be not larger than 70 mλ. Thus, when considering the above-described third-order coma aberration generated by the inclination of the disk and the third-order coma aberration generated by the manufacturing error and the assembly error, it is impossible to record and reproduce data in this system.
When the lens having a numerical aperture of larger than 0.6 is designed by the design technique as described above, a satisfactory design is not possible because the aberration due to the decentration and the aberration due to the abaxial light generally are in an inversely proportional relationship. In other words, the lens becomes very difficult to manufacture or assemble. Also, when an optical disk is inclined with respect to an optical axis due to the warp of the disk or the like, a great deal of the third-order coma aberration is generated, so that it becomes impossible to record or reproduce data in this system.